1. override
String debugDescribeChildren(String prefix)

Returns a string describing the current node's descendants. Each line of the subtree in the output should be indented by the prefix argument.

Source 

@override
String debugDescribeChildren(String prefix) {
  if (firstChild != null) {
    String result = '$prefix \u2502\n';
    ChildType child = firstChild;
    int count = 1;
    while (child != lastChild) {
      result += '${child.toStringDeep("$prefix \u251C\u2500child $count: ", "$prefix \u2502")}';
      count += 1;
      final ParentDataType childParentData = child.parentData;
      child = childParentData.nextSibling;
    }
    if (child != null) {
      assert(child == lastChild);
      result += '${child.toStringDeep("$prefix \u2514\u2500child $count: ", "$prefix  ")}';
    }
    return result;
  }
  return '';
}